Mix flour, salt, baking powder, and cinnamon in a separate bowl; add to apple mixture and stir just until evenly combined into a thick batter. Pour batter into the prepared loaf pan.

Bake in the preheated oven for 45 minutes. Cover loaf pan with aluminum foil; continue baking until a toothpick inserted into the center comes out clean, about 15 minutes more. Cool in the pan for 10 minutes before removing to cool completely on a wire rack.